Process for extracting silver from ores, concentrates, or other materials



AP 16, 1940- H. E. LEE ErAL PROCESS FOR EXTRACTING SeILVER FRQM CRES, CONCENTRATES; 0R OTHER MATERIALS Filed Apr-i1 25, 193,8

Patented Apr. 16, 1.940

PROCESS FOB EXTRACTING SIINER- FROM gRES. (ONCENTRATES. OR OTHER MA- Hamm Eugene ue nu Burton neben! Meir, Kelle, Idaho, assignors to Bunker Hill &

Sullivan & concentrating Company,

Kellogg, Idaho, a. corporatiou 'application April z3. 1938,. serial No. 203,968 vs emma. (or, '1s-11s)7 Our present invention relates to ,an improved process for extracting silver from ores', concentrates, or other materials, and more particularly to the hydro-chemical extraction and recovery of silver, or silver in conjunction with other ele'- ments from materials which contain varying proportions of silver in the metallic state, 'or in which the silver is-associated with constituents,

1- such as ferrous sulphate, or the lower oxides of nable to the usual direct leaching methods, or,

becauseof the nature and concentration of associated constituents, to direct smelting methods, such'as silver and gold bearing lead-copper sulphide mixtures containingvarying proportions of 1ron, antimony, arsenic, zinc, bismuth, selenium, tellurium, and other objectionable impurities.

The process in general comprises the roasting of the ore, concentrate, or other material, to a calcine in proper manner as hereinafter fully described, delivering the calcine to a neutral leach circuit, or if desired to an acid leach circuit or combinations of both, in which a balance Aof ferric ion concentration over ferrous ion concentration favorable to the solution of metallic silver is maintained, preferably by the use of suitable oxidizing agents such as potassium permanganate, or manganese dioxide; thickening, washing, and filtering the leached solids; subjecting the resultant solutions to operations for precipitation of the dissolved silver; and the recovery ofthe precipitated silver.- 3

The hydro-chemical extraction of silver from.

materials in which silveris soluble or has been..

rendered soluble, to neutral or acidied solutions is decreased in proportion to the concentration of associated substances such as ferrous sulphate, the lower oxides of iron or copper, metallic copper, or other constituents capable of precipitating silver as metallic silver from solution.

It has been determined that the concentration of such detrimental constituents may be eifectively controlled by adequate heat treatment prior to leaching; or by the use of suitable oxidizing agents, such as potassium permanganate' or manganese dioxide,'during the leach; as well as by combinations of both these steps; thus allowing the silver extractions at substantially higher percentages than those otherwise obtainable. It

- has also been determined that lowered silver exiron or copper, metallic copper, or other such, substances capable of precipitating it to thev Y tractions, due to the presence of metallic silver, 'either as a constituent of the original product, or formed during. subsequent treatment operations, may be substantialy increased by the use of ferrie sulphate, either directly, as an addition 5 agent, or formed through the use of addition lagents, or formed as anfinherent product of the operation, in conjunction with the maintenance of a balance between the ferric ion concentration and the ferrous ion concentration, preferably by the use of suitable oxidizing agents such as potassium permanganate, or manganese dioxide which favors the following reaction to proceed to the left: l

-AgiSO4+2FeSO4r22Ag|Fe2(S04)3 Application o`f the principles and procedure l comprising this invention enables the attainment of substantially higher silver extraction than would otherwise result from the treatment sponge, by precipitation on an electro-positive metal, or by electro-deposition vas metallic silver, 0

'the silver barren solution being either wasted, 'Y

circulated as return solvent.' or subjected to subsequent treatment for the recovery of other ,values contained in the solution.

' In the accompanying drawing, we illustrate a diagrammatic view of the iiow sheet of a suitable apparatus by which the following process may be carried out in a particular application to a material in 'which the silver is associated with copper. This ow sheet is included as a part of the disclosure to illustrate the various stages of the process and the direction of the flow taken by the solutions and solids. -The drawing in conjunction with the application of the' process to one 45 specic ,type of material will afford a full understanding of the nature of the invention depicting its full range and the improved results obtained thereby. l y

In a preferred method of caryingout our process, particularly adaptable to sulphide lores or concentrates, the silver content, or 'silver in conjunction with other components, may be brought to a solublestate by roasting at A the material under suitable conditions of time and temperature as to render the maximum percentage of silver as the water soluble silver sulphate; render the maximum percentage of other desired constituents to forms soluble in neutral or acidied solutions; to prevent or tendl to prevent the formation of insoluble compounds of silver or other constituents deslred'leached; to insure the presence of the minimum concentrations of ferrous sulphate, metallic copper, the lower oxides of iron and copper, or othercomponents thatv act as silver precipitants in subsequent leaching operations; to prevent excessive sinterng of the charge when appreciable quantities of low fusion point constituents, such as lead and antimony sulphides are present; and to eliminate the maximum percentage of undesirable volatile impurities. f The material, when properly calcined, may be delivered to the leaching circuit and the solution of silver effected continuously or in batches, in either case utilizing one or more leach steps, depending upon the concentrationofsilver and the nature of associated constituents." lIn the treat- .ment of silver rich materials, bearing relatively high proportions of silver sulphate, a preliminary leach at C in neutral solvent may be preferable. Whilea single neutral leach, followed by suitable decanting and washing operations, may sufiice, usually the presence of residual metallic silver makes desirable its subsequent treatment in acid solutions, where the solvent power of ferric sulphate for metallic silver is much more eiective. In cases where the silver content of the material being treated is associated with other constituents, such as copper, the desired additional silver leaching stages may be effected in conjunction with subsequent operations for the extraction of copper.

The effect of silver preclpitants, during the leaching operations, may be circumvented and the solution of metallic silver enhanced through the maintenance of a balance of excess ferric ion concentration over ferrous ion concentration, 'iuclllit as to favor the reaction to proceed to the the ferric sulphate being added as an addition agent, or preferably formed from iron inherent to the system, by the use of oxidizing agents, such f as manganese dioxide, alone, or in combination with air, potassium permanganate, or other suitable oxidizing agents known to the arts.

In operations for the combined extraction of silver and copper from silver rich materials, when an initial neutral leach for silver is preferable, the neutral solvent may be maintained of sumcient copper concentration, without appreciably affecting the solubility of the silver, to allow its blending with the solution resulting from the low acid to neutral leach iiowing to the continuous silver precipitator.

In such cases, as illustrated by the accompanyving drawing, the calcined material may be env tered into the neutral leach system C, throughy a classifier B which is set in closed circuit with a suitable grinding appliance. 'I'he classiiler overilow is continuously conveyed to aseries of agitators and treated under suitable conditions of time and temperature for the solution of the major portion of the silver; then the pulp consolution and suspended silver sponge being conveyed to a thickener, or thickeners F, that discharge the thickened silver pulp through an agitator G to intermittent washingH and nitration I. The silver barren overflow may be passed through an additional settler J and thence to storage K for circulation as neutral leach solvent or to subsequent steps for the recovery of copper.

The thickened, partially leached roast from the neutral leach may bepumped to a series of agitators S with partially neutralized acid solvent, the proportion of sludge being in excess of that required to neutralize the free acid. By the addition of suflicient excess sludge and the use of adequate time and temperature conditions, preventing solution short circuits by the use of a sufficient number of agitators in series, the solution may be thoroughly neutralized and rendered, for all practical purposes, free oi' objectionable impurities.

`From the last of these agitators the pulp, consisting oi' neutralized silver .bearing copper solutionl and suspended solids, passes to a decanting thickener or thickeners T, in which 'the solution is separated from the solids. The thickener overflow is treated in a manner identical to that described above for the solutions resulting from the neutral leach. The silver barren solution from both the above steps, may be treated for the recovery of copper as copper sulphate by evaporation, or for the recovery of copper as cement copper 4by precipitation on an electropositive metal, or, as'shown by the accompanying specinc illustration, it may be conveyed to electrolytic cells L for the regeneration of acid and the eelctro-depositin of copper as metallic copper.

'Ihe regenerated electrolyte, or newacid M, in cases where the copper is recovered as copper sulphate or cement copper, may be fed to the iirst of a third series of agitators N along with the sludge underiiow from the low acid to neutral thickener or thickeners, the proportion of acid solvent utilized being such that the leached pulp leaving the last leaching agitator of this series will contain free acid, in 'order that the resulting solids may be free or substantially free from all undissolved soluble silver and copper.

The leached pulp then passes to the rst of a series of decanting thickeners 0, where the dissolved valuesv are washed by Acounter-current decantation. This is done in a manner known tc the arts in which the thickeners are arranged in a series of steps, one above the other, the first ol' these thickeners O, which receives the leached pulp, being the lowest and the nnal, or'discharge thickener P being' at the highest level of the series. Y

In this arrangement the underflow of the first washing thickener is pumped to the well of the second, andso on to the well oi' the last washing thickener. Wash solution R is added to the well of the last thickener and overtlows through-the series, counter-current to the advancing solids, overflowing from the first cf the series as partially neutralized electrolyte, or acid, and conveyed to the ilrst oi' the low acid to neutral agitators S. f

The underow from the last of the series of thickeners is pumped to a nlter Q where it is "f ed in wash solution R and dewatered. The filtrate from this operation is pumped to the well oi.' the last washing thickener P and the cake discharged.

In a specinc case, applicableto the general of its dsilver' and copper was discharged in a form Y .preferred treatment illustrated by the drawing, about 250 tons of material containing 10.5 per cent moisture and the following range of compisition: 1650 oz. Ag/ton; 20Vper cent copper; l1- per cent lead; l2-per cent iron; 5 per cent insoluble; 0.25 per cent bismuth; 25 per cent sulphur; 18 percent antimony; 1.5 per cent arsenic; was calcined at the rate of 35 to 40 tons per 24 hours in a Wedge type multiple hearth furnace. A calcine was produced which, when subjected to the leach conditions outlined above, resulted in the extraction of over 96 per cent of the silver, the silver being quickly recovered in a form suitable for direct cupellation and' the copper as cathode copper of purity equal to that produced in geenral by the prior methods. The resulting residue, reduced in mass and stripped oi' the bulk more readily amenable tofurther treatment by smelting or other methods, for the recovery of residual values contained.

The roastingy step was conducted during the early stages of the operation at a temperature ranging from 400 C. to 450 C., and during the latterstages at approximately 600 C. The cal- -cined discharge was screened to a desirable size,

-and the oversize was crushed and returned for system through a classier with neutral solvent,

as depicted on "the accompanying diagram. The coarse material was raked to a suitable grinding appliance which was set in closed circuit with the classier. The classifier fines continuously overiiowed to an agitator or agitators where sultable time and temperature conditions were allowed for the solution of the bulk of the silver,-

the solvent and suspended solids continuously discharged fromthe agitation operation to a decanting thickener, or thickeners, the overiiow ofv which 'passed continuously to and through a precipitator in which the solution was freed of its silver content, and'thence to a decanting thickener. or thickeners, wherein the suspended silver solids vwere thickened, the claried silver free solution going to a general copper sulphate l storage and the underflow conducted through an agitator to a nlter where it was washed and the washes advanced until of suiiicient concentration to iiow to the general copper sulphate storageunits, and the washed sponge cake discharged in a form suitable for direct cupellation.

'Ihe underflow sludge ofthe initial leaching operation was conducted continuously to the first of a series of agitators where it met partially neutralized relectrolyte in proportions and under conditions described in foregoing considerations, passing through and discharging from the last of the series to a decanting thickener or thick-' eners, the overflow of this thlckener or thickeners joining the overow from the initial leach thickener or thickeners and the underow sludge being conducted to the first ofthe nal'series of. agitators along with regenerated acid of P20901'- tions to provide free acid in the overflow of the last 'agitator of the series. l

The acidifled discharge of the final series of agitators was conveyed te a series. of countercurrent decanting thickf-ners where the pulp was advanced counter-einrently against solution of' decreasing acid and value content and finally pumped to a iilter where it was iiltered and discharged, the filtrate being pumped to the well of the last thickener of the counter-current decantation series.

The solution in the general copper sulphate storage unit was utilized as return solvent for the initial silver leach and as feed to the electrolytic unit wherein the copper was recovered by electro-deposition as metallic copper-and the acid solvent'regenerated and returned to the cycle through storage units to the ilnal series of agitators. A desired portion of this general storage was bled from time to time to onset excessive acid generation or to control the excessive build-up of impurities in solution by evaporation of portions and the recovery of copper as copper sulphate, or by passing over an mate extraction is not appreciably greater than that obtained under similar time and temperature conditions with unground material.

The temperature and time requirements throughout the leaching operations are subject to considerable variation; however, temperatures in the rangeof 40 C.60 C. allow-V preferable operating conditions. mth such a range of op erating temperatures adequate results may be obtained with contactperiods during the various leaches of the following order, (1) initial silver leach, 0,754.5 hours, (2) neutralization leach, 3'6 hours, (3) nal acid leach, 2-5 hours.

Throughout the foregoing procedure, the effect of silver precipitants was circumvented by, (l)

roasting under"y suitable predetermined time and temperature conditions as to favor the dissociation of ferrous sulphate and aiford the maximum economical opportunity for oxidation of metallic copper and the lower oxides of iron and copper. (2)v maintenance, by the use of oxidizing agents such as potassium permanganate where rapid action and freedom of the solution from solid contaminant was desired, and manganese dioxide, in conjunction with air, where more time was available and the presence of solid contaminant not objectionable, of a balance of ferrie ion concentration over ferrous ion concentration, utilizing the iron components inherent tothe system. which continuously favored the solution contacted the partially leached calcine being fed to the low acid to neutral system. The quantity ofl oxidizing agent required for high extractions is obviously a function of the concentration of metallic silver, or of silver precipitants, in the material being treated; increasing proportions of these constituents increasing the required consumption of oxidizing agent or agents.

In the specific application under consideration, treating analogous` materials, the consumption of oxidizing agent required for analogous extractions varied from lbs, Mn02 and 4 lbsjof KMnO4 per ton of material, exposed to less suitable roasting conditions, to approximately 4 lbs, of MnOz and 2 lbs. of KMnO4 per ton of material exposed to more suitable roasting conditions. In actual analogous treatment of identical calcine, in one case maintaining a suitable balance of excess ferrie ion concentration over ferrous ion concentration, by means of the forementioned oxidizing agents, silver extraction of over 96 per cent was obtained as compared to silver extraction approaching '10 per cent when no control was exerted and the ferrous ion .concentration allowed to assume unfavorable proportions. A

From the foregoing description it will be ascertained that substantially higher hydrochemical extraction of silver, or of silver in conjunction with, other constituents, than otherwise obtainable, may be effected from ores, concentrates, or other materials, containing varying proportions of silver, anda wide range of other constituents, by preliminary heat treatment under suitable time and temperature conditions, or by leaching in neutral or acid solvents, or combinations of both, in which a favorable balance of excess :ferrie ion concentration over ferrous ion concentration is maintained by the use of suitable oxidizing agents such as m04 of MnOz, or by combinations of these procedures; the silver being recovered as metallic sponge by precipitation on an electro-positive metal, or as metallic silver by electrodeposition,and the silver barren solution wasted, or circulated as return solvent or advanced to subsequent operations for the extraction and recovery of other associated constituents. v

I It is evident that the above procedure and i1- lustrations maybe modied to meet the particular needs of the operations to which it is applied, and is not intended to restrict it to the particular examples given. The terms used .in

describing this invention have been used in their descriptive sense and not as terms of limitation and it is intended to include within the scope of the appended claims all equivalents of thevprocedures described.

Having thus fully desc ,'bed our invention,

w what we claim as new and desire to secure by Letters Patent is:

1. A process for extracting silver from ores, concentrates or other materials, which consists in treatment to form a calcine, then leaching the calcine in solutions containing an excess of ferric ion concentration over ferrous ion concentration such as to favor the solution of metallic tivelyconverted to its sulphate by calcination,

then leached as such, high extractions being obtained by converting and maintaining all associated constituents in their higher oxide forms.

3. 'I'he hydrochemical process for the extraction and recovery of silver from refractory iron lbearing sulphides in which the silveris selectively converted to its sulphate and the associated constituents to their higher oxide forms rby calcination, as described, and the calcine leached in solutions containing an oxidizing agent, maintaining the oxidizing agent to favor the reaction Fez(SO4)3+2Ag- AgzSO4+2FeSO4 by holding the ferrous ion concentration at a minimum.

4. The hydrochemical process for the extractionl and recovery of silver from refractory sulphides in whichlthe silver is selectively converted to its sulphate and the remaining associated constituents to their higher oxide forms by calcination, as described, and the calcine leached for the extraction of silver as silver sulphate in vsolutions containing an oxidizing agent, maintaining the oxidizing agent to hold both the solids and dissolved salts in their higherl oxide states.'

5. The herein described process for the extraction and recovery of silver from refractory sulphides which comprises calcination for the formation of silver sulphate and the higher oxides of the associated constituents, leaching the sulphate silver, then leaching the residual solids in sulphate solutions containing iron and an oxidizing agent, maintaining the oxidizing agent to favor the solution of metallic silver by holding the ferrous ion concentration at a minimum.

6. The herein described process for the extraction and recovery of silver from refractory materials which comprises calcination to form silver sulphate and the higher oxides of the associated constituents, washing the calcine free of sulphate silver, then leaching the residual solids in iron bearing sulphate solutions containing an oxidizing agent, maintaining the oxidizing agent to hold the ferrous ion concentration at a minimum, and recovering the dissolved silver as sponge.

HAROLD EUGENE LEE. BARTON ROBERT MUIR. 

